Biological fate of methenamine in man. Absorption, renal excretion and passage to umbilical cord blood, amniotic fluid and breast milk.

Methenamine hippurate was administered orally as tablets or granules to healthy volunteers. Plasma concentrations of methenamine reached a maximum 1--2 hours after a single dose and then declined with a half-life of about 4 hours. The apparent distribution volume was similar to that of total body water. Renal clearance of methenamine was somewhat lower than that of creatinine. In cross-over experiments over six days, methenamine recovered in the urine corresponded to about 80 per cent of the dose given per 12 hours, slightly lower values being obtained from granules than from tablets. The efficient renal elimination of methenamine was confirmed in similar studies on patients post-operatively. Methenamine hippurate was also given to healthy pregnant women during labor, a few hours before expected delivery. Methenamine was found to pass the placental barrier. The concentration of methenamine in umbilical cord plasma was low but reached the level in maternal plasma after about 4 hours. In amniotic fluid the methenamine con centration was low and varying. No correlation was obtained to the maternal or umbilical cord plasma levels. The methenamine concentration in breast milk was of the same magnitude as in maternal plasma. It is concluded that methenamine may be safely given to pregnant and lactating women with respect to the ellbeing of the child.